1. Field of the Invention
The present invention relates to an inverter driving circuit which drives a plurality of electric-discharge lamps, and in particular, relates to an inverter driving circuit which simultaneously drives cold cathode fluorescent lamps (CCFL) for back lighting of various display panels used for a Laptop computer, a liquid crystal television, and the like.
2. Description of the Prior Art
An inverter driving circuit in which a booster transformer is mounted so as to drive a CCFL for back lighting of various display panels is known conventionally. In addition, recently, according to requests of upsizing of display panels, an inverter driving circuit which can simultaneously drive a plurality of CCFLs attracts attention. In such an inverter driving circuit, separately from the above-mentioned booster transformers, a balance transformer having special construction for making currents, which flow into a plurality of CCFLs, coincide mutually, for example, one which is described in Japanese Unexamined Patent Publication No. 2006-12781 is used.
Nevertheless, such a balance transformer has a limit in miniaturizing, and since it is generally arranged in a preceding stage of CCFLs, it has become an obstruction for promoting miniaturization of a display panel by securing the installation space (substrate area).
From such a situation, development of inverter driving circuits having the same function as that of a balance transformer has been also tried by devising circuit construction. For example, a circuit in which a series resonant circuit is connected to a preceding stage of primary windings of booster transformers as described in Japanese Unexamined Patent Publication No. 2006-59761 is known. In this way, it becomes possible to make approximately equal values of currents which flow into respective primary windings without using a balance transformer by connecting a series resonant circuit to a preceding stage of a plurality of primary windings.
Nevertheless, in the circuit construction described in the above-mentioned Japanese Unexamined Patent Publication No. 2006-59761, respective primary windings of the transformer arranged in a subsequent stage of the series resonant circuit are mutually connected to a power supply in series. Hence, since an applied voltage to each primary winding of the transformer is divided according to the number of the primary windings, and, it becomes necessary to enlarge a winding ratio. Hence, it was said that it was problematic construction in terms of efficiency.